The present invention relates to a fastening system including pull type fasteners, comprising a pin and a sleeve, for manufacturing shipping containers and the like.
Shipping containers are frequently of a construction including a hard metal exterior layer and one or more interior layers of softer materials such as plywood, fiberboard, etc. In fastening the layers together pull type fasteners can be used of a type generally shown in the U.S. Pat. No. 3,515,419 issued to C. W. Baugh on June 2, 1970.
The pull type fastener includes a pin and a sleeve and is set by applying a relative axial force between the pin and sleeve with the excess shank of the pin being severed upon completion of the installation. With that type of fastener, it is desirable that the component engaging the interior surface have a relatively large head to provide good load distribution to inhibit crushing. It is also desirable that the set fastener have a relatively low profile and hence smooth surface on the interior side in order to maximize the interior load carrying volume and to avoid snagging of the container contents on fastener protrusions. It is also desirable that the fastener provide a watertight seal and inhibit tampering, i.e. fastener removal. It is also desirable that the fastener have a large bearing, low profile head to engage the outer container surface to support high clamping loads and resist vibration. Likewise it is important that installation loads and pin break shock loads be kept to a minimum to avoid the effects of pin bounce loosening the integrity of the joint.
In some constructions, such as in the Baugh patent supra, a combination mechanical lock and friction lock is used to hold the pin and sleeve together. Speaking generally this is a functionally satisfactory design. However, the design is somewhat complex and thus expensive, e.g. an extended sleeve, a carefully constructed convex shaped undersurface on the pin head and a multisectioned pin construction. It is particularly grip sensitive in that it can not accommodate a wide variation in thickness of the workpieces being secured together by the fastener. Further the friction lock between the sleeve and straight splined pin portion is only supplementary to the mechanical lock formed by turning out the ends of the sleeve to overlap the bore at the exterior of the workpiece. And last, the pin break loads are fairly high since so much metal is being moved, e.g. the sleeve expanded at the friction lock and flared at the end to form the mechanical lock. A high pin break load by itself is not particularly undesirable. However this results in high pin bounce, or reaction force, when the fastener is brought to final clamp up and the disposable pull portion of the pin is pulled off by a pure tensile load. This pin bounce or reaction force can result in lower, unsatisfactory, final clamp up loads of the fastener relative to the workpieces.
As an improvement over the aforementioned Baugh design, the assignee of the present invention developed the fastener shown in FIGS. 7 and 8 as prior art, which is more fully disclosed in abandoned U.S. patent application Ser. No. 701,347, filed Feb. 13, 1985 the disclosure of which is incorporated herein by reference. It included a helically splined pin portion for the friction lock and an internally formed mechanical lock at the rearward end of the splines formed by a radially extending annular lock shoulder on the pin adapted to engage and deform the lead end of the sleeve into a lock pocket. It offered the advantages of greater grip range accommodation and slightly less expense due to the shorter sleeve length. But, pin break loads remained fairly high, as in Baugh. Consequently, pin bounce remained a significant concern. Furthermore, despite the theory that on installation the pin would rotate slightly relative to the pin as the sleeve overcame the helical splines, this in fact did not occur, at least not with any degree of reliability. This is due to several reasons, not the least of which is that with the installation equipment designed for the fastener (i) the gripping pressure of the tool jaws was too high to allow the pin to rotate and (ii) the friction between sleeve head and tool anvil was too high to allow the sleeve to rotate. Finally, this particular fastener was designed such that, as in Baugh, the sleeve would be radially expanded by the pin spline portion into an interference fit with the bore. This requires a sleeve material which, although softer than the pin, is relatively hard and this in turn was found to result in the sleeve material resisting flow into the helical grooves of the pin spline portion. In some cases the sleeve material was radially sheared since the pin and sleeve could not rotate relative to one another as noted above.